1. Field Of The Invention
This invention is generally concerned with the use of freezing conditions in order to accomplish a degree of separation of a solvent and a solute from a solute/solvent liquid solution. The herein disclosed processes have many useful ecological and industrial purposes, but they are especially well suited to converting sea water, brackish water, etc. into potable water, concentrated brines and purified solutes.
Numerous methods and/or systems have been developed which employ freezing conditions to produce potable water, but they have not experienced wholehearted economic and/or technical acceptance. For example, one of the principal problems inherent in most currently employed desalination processes which employ freezing conditions is their inability to produce potable water at economically acceptable costs. That is to say that not only must a desalination process be able to produce large volumes of potable water, it also must be able to do so at locally acceptable costs. Again, cost considerations are particularly important to this art because many areas of the world having the most acute potable water shortages also are characterized by very low income levels.
In most processes employing freezing conditions to produce potable water, sea water is simply frozen to produce an ice product and/or it is flash-frozen to produce a water vapor product and a slurry of ice and brine. Both of these freezing techniques require a great deal of expensive refrigeration capacity. The flash freezing techniques are especially expensive because they employ vacuum/freezer apparatus wherein a vacuum is created to evaporate the water. This is accomplished by the expenditure of a great deal of mechanical work. The resulting water vapor must be continuously removed from the vacuum/freezer apparatus and then must be condensed to water by another large expenditure of mechanical work. This all goes to say that the refrigeration, vacuum-creation and condensation steps of such processes each require considerable amounts of mechanical and/or electrical energy which can only be obtained at high fuel costs and/or high capital equipment costs. Consequently, freezing and flash-freezing processes to produce potable water have not been widely accepted as economically viable methods of producing potable water.
2. Description of the Prior Art
Some representative freezing techniques heretofore suggested or used for the production of potable water from sea water are taught in the following patent references.
U.S. Pat. No. 4,236,382 teaches a desalination process wherein a deaerated, ice water solution is first flash vaporized under a highly reduced pressure to form a low pressure water vapor brine and ice crystals. The ice is purified in a counter-washer and then melted inside heat conductive conduits under high pressure. The low pressure water vapor is then desublimed to form a desublimate (ice) on the outside of certain conduits employed in this process. This particular process employs the latent heat of desublimation of the desublimate in order to supply the heat needed in the ice making portion of the operation.
U.S. Pat. No. 3,664,145 teaches a method for separating a solvent (e.g., water) in substantially pure form from a solution (e.g., sea water) wherein a vacuum freezer is employed to produce vapors and a slurry of solvent and solute. The product materials are separated by various novel, but complex, mechanical steps which form a large part of this particular patent disclosure.
U.S. Pat. No. 3,070,969 teaches a process which also employs vacuum freezing conditions in order to separate dissolved salts, such as those found in sea water, from a solvent solution. The sea water is vacuum frozen by a novel arrangement of equipment in order to both concentrate the solute salts contained in the liquid component of the solution and in order to collect the solvent component of the solution in the form of a frozen solvent material (e.g., ice).
U.S. Pat. No. 3,214,371 teaches a desalination process which is based upon formation of large ice crystals in brine through the use of certain water clathrate substances such as propane hydrate. The ice crystals are separated from the brine (and from the clathrate substance) and then melted in a process which employs the latent heat absorbing capacity of the ice to further promote formation of a hydrate produced from a brine and water clathrate feedstock.
Again, the principal drawback with most of the above noted freezing or vacuum freezing methods is that the amount of pure water which can be produced thereby is directly proportioned to the size and efficiency of the mechanical compressor(s) needed to produce the refrigeration and/or vacuum freeze conditions necessary to carry out such processes. Again, even under the best of conditions, the economic costs of running such compressors is enormous. Consequently, the extremely high costs of vacuum freeze processes such as these usually can only be justified in processes wherein the value of the end product (e.g., blood plasma and freeze-dried coffee) is very high. They are not normally used to produce potable water, not because they do not work, but rather because they are prohibitively expensive to build, maintain and operate. Moreover, in those cases where vacuum freezing has been used to produce potable water (high costs notwithstanding), the mechanical compressors needed to carry out such processes have been subject to scaling, corrosion and mechanical failure due to the deleterious effects of the salt content and microbiology of sea water.
In response to all of these technical problems and economic circumstances, the herein disclosed processes are intended to provide certain methods, systems and apparatus for separating a solvent (e.g., potable water), in substantially pure form, from a solute/solvent solution (e.g., sea water) at substantially lower costs than those which can be obtained by conventional refrigeration and/or flash freezing methods. These advantages are obtained through the use of a three phase separation process which, among other things, does not require any compressor equipment. It should also be noted that small, conventional, mild steel motive equipment, low pressure vessels and heat exchangers will suffice for use in the processes of this patent disclosure. Moreover, the labor and maintenance requirements of these processes are much less than those associated with compressor equipment.